Internal combustion engines are commonly used to provide power for motor vehicles as well as in other applications, such as for example for lawn mowers and other agricultural and landscaping equipment, power generators, pump motors, boats, planes, and the like. For a typical driving cycle of a motor vehicle, the majority of fuel consumption may occur during low-load and idling operation of the vehicle's internal combustion engine. Similarly, other uses of internal combustion engine may also be characterized by more frequent use at a power output less than that provided at a wide open throttle condition. However, due to mechanical friction, heat transfer, throttling, and other factors that can negatively impact performance, spark ignition internal combustion engines inherently have better efficiency at high loads and poorer efficiency at low loads.
Efficiency at lower engine loads can be improved in some instance by increasing a compression ratio of the engine. The compression ratio is a measure of the degree to which an air-fuel mixture is compressed before, ignition that is defined as the expanded volume of the engine combustion chamber divided by the compressed volume of the engine combustion chamber. A high compression ratio in a standard Otto cycle engine generally results in the piston performing a longer expansion in the power stroke, and consequently more work, in comparison to the same engine running at a lower compression ratio. Compression ratios of gasoline powered automobiles using gasoline with an octane rating of 87 typically range between about 8.5:1 and 10:1.
The maximum compression ratio achievable by an engine can be limited by uncontrolled advanced (i.e. prior to an intended timing) ignition of the air-fuel mixture at high temperatures, a problem commonly referred to as engine knock. Knock can occur as a result of disassociation of the fuel into more easily combustible molecular fragments when the mixture is exposed to high temperatures for a sufficiently long period of time. The high temperature exposure can result in these fragments initiating an uncontrolled explosion outside the envelope of the normal combustion. For example, auto-ignition typically occurs prior to the piston reaching the top dead center (TDC) position of a compression stroke, so in some cases knock can occur before the piston passes TDC to start the expansion stroke. Auto-ignition can also occur on the expansion stroke as the end gas is heated and compressed by the already burned mixture so that pockets of the combustion mixture ignite outside of the normal combustion envelope. Engine knock causes audible and potentially damaging pressure waves inside combustion chamber. Knock is a specific problem associated with the more general issue of auto-ignition. In this document, auto-ignition refers to instances in which the ignition occurs independently of when the spark is fired, as in homogeneous ignition or a burn initiated by a surface ignition prior to the spark event.
A variety of factors in addition to high compression ratios can affect the occurrence of knock in particular and auto-ignition in general. In general, low octane gasoline may spontaneously ignite at lower temperatures than high octane gasoline. Hot wall or piston temperatures in engines can also tend to increase the heating of the air-fuel mixture, thereby increasing a tendency of the fuel to auto-ignite, as can localized hot spots, such as around the exhaust valve, which may cause localized heating of the air-fuel mixture and knocking in the area of the hot spots. A fast burn rate of the fuel-air mixture, for example due to high turbulence, which promotes good mixing and rapid burning of the fuel, can reduce the likelihood of spontaneous ignition. However, high inlet flow field turbulence can also increase the temperature rise in the inlet air-fuel mixture, which increases the likelihood of spontaneous ignition. Increasing the quantity of fuel in the mixture up to stoichiometric can increase the energy released and hence the pressure and temperature of the end gas, which can affect the tendency to knock. Advanced ignition timing can also generate high peak pressures and temperatures, thereby contributing to a tendency for auto-ignition under some conditions.
Many conventional internal combustion engines are typically configured for a four-stroke Otto cycle, an idealized version of which is illustrated by the chart 40 of FIG. 1A. As shown, the four-stroke Otto cycle includes an air/fuel inlet stage 50, an isentropic compression stage 52, a constant volume combustion stage 54, an isentropic expansion stage 56, a blowdown stage 58, and an exhaust stage 60. The piston compresses the mixture during the compression stage 52 to the same degree that it expands during the power stage 56. The Otto cycle is generally characterized as having its best efficiency at high loads with substantially reduced efficiency at lower loads (e.g. while operating a throttled condition). Pumping loses against the throttle can also be significant. The symmetry of an Otto cycle can also lead to limited efficiency. In an Otto cycle engine, a throttle is typically used to limit the airflow for part-load operation. The throttle restricts the airflow into the manifold so that the engine pulls in air from this reduced pressure region. So the work to pump the air into the engine is typically higher than if the valves had been used to limit the airflow.
In contrast, the Atkinson cycle can provide a higher efficiency than the Otto cycle by utilizing an asymmetric cycle that reduces pumping work. When an engine is operated in an Atkinson cycle, the effective air/fuel compression stroke is shortened relative to the power expansion stroke. This may be accomplished, for example, by keeping the inlet valve closed for a portion of the air/fuel inlet stroke, thus reducing the mass of the air-fuel mixture admitted for the compression stroke. The geometric compression ratio is such that this limited amount of charge is compressed near the limits imposed by the octane rating of the fuel. The compressed mixture is then ignited and expanded through a expansion stroke that is longer than the compression stroke. The chart 61 of FIG. 1B illustrates a first idealized version of the Atkinson cycle. In this example of an early intake valve closing Atkinson cycle, the air-fuel mixture may be drawn in at stage 62 without a change in pressure until a volume V0 of mixture is admitted. At that point, the inlet valve is closed and a second part 64 of the inlet stroke continues with no more mixture being admitted. The pressure in the chamber drops during the second part 64 of the inlet stroke at this point because the volume continues to expand with no additional air being added, The mixture is compressed during an adiabatic stage 66, combusted at constant volume in a combustion stage 68, and adiabatically expanded in a power stage 70. The exhaust gases are withdrawn in an exhaust stage 74 following a blowdown stage 72. The chart 80 of FIG. 1C illustrates a second idealized version of the Atkinson cycle. In this example of a late intake valve closing Atkinson cycle, the inlet valve is left open throughout the inlet stroke 82 and also through the first part 84 of the compression stroke until the inlet valve is closed. The volume of mixture V0 remaining in the combustion chamber after the closing of the inlet valve is compressed during an adiabatic stage 86, combusted at constant volume in a combustion stage 88, and adiabatically expanded in a power stage 90. The exhaust gases are withdrawn in an exhaust stage 94 following a blowdown stage 92. Thus, in an Atkinson cycle, the expansion stages 70 and 90 is increased relative to the compression stages 66 and 86. The Atkinson cycle increases efficiencies at lower loads by extracting more work out of an expansion stroke for a given compression ratio.
The Atkinson cycle is generally unable to provide high power densities for high load engine operation. Because of power density limitations, however, the Atkinson cycle in conventional engines is usually used only for low loads. Variable valve timing and variable compression ratio can be used to make an engine operate in the Atkinson mode at low power and in the Otto cycle, or symmetric mode, for high power. One approach to achieve this type of operation is a late inlet valve closing (LIVC) strategy, such as for example that shown in FIG. 1C and discussed above.